Handle empty items in item lists

common/src/main/java/com/viaversion/viaversion/protocols/protocol1_20_5to1_20_3/rewriter/StructuredDataConverter.java

@@ -755,6 +755,7 @@ public final class StructuredDataConverter {
         final ListTag<CompoundTag> itemsTag = new ListTag<>(CompoundTag.class);
         for (final Item item : items) {
             final CompoundTag savedItem = new CompoundTag();
+            if (item != null) {
                 final String name = toMappedItemName(item.identifier());
                 savedItem.putString("id", name);
                 if (backupInconvertibleData && name.isEmpty()) {
@@ -767,6 +768,10 @@ public final class StructuredDataConverter {
                     writeToTag(data, itemTag);
                 }
                 savedItem.put("tag", itemTag);
+            } else {
+                savedItem.putString("id", "air");
+            }
+
             itemsTag.add(savedItem);
         }
         tag.put(key, itemsTag);